Fired FBI Director James Comey's blunt testimony about Donald Trump's demand of loyalty and attempt to quash the bureau's investigation into former National Security Adviser Michael Flynn was historic. Never before has the nation's ex-top-cop testified in an open Senate hearing that the sitting president is a liar who acted to "defame" the FBI.

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In his Senate committee hearing Thursday, the former FBI director said he believes the president to be a liar, among other things

Both in his vivid written statement and his answers of senators' questions, Comey pulled back the curtain on Trump's reckless – and potentially unlawful – behavior. But Comey's testimony also left open troubling questions, including many that the former FBI director appeared to have answers to, but told senators he could not speak to in an unclassified setting.

Comey's testimony offered five big takeaways.

First: From their initial meeting, Comey pegged Trump as a fundamentally dishonest human being. He told senators that he felt compelled to create an evidentiary record of his interactions with the president: "I was honestly concerned that he might lie about the nature of our meeting," Comey said, "and so I thought it really important to document." Comey later added, "I knew that there might come a day when I would need a record of what had happened, not just to defend myself, but to defend the FBI." Comey was so disturbed by his private interactions with the president that he found relief in the news Trump might have recorded their meetings. "Lordy, I hope there are tapes," Comey testified, later imploring the president, "Release all the tapes, I'm good with it."

Second: Comey rejects any efforts by the president to cast doubt on Russia as the perpetrator of the hacks against the DNC, the DCCC and other political operatives, whose communications were pushed out on WikiLeaks to damage the Clinton campaign. "There should be no fuzz on this whatsoever," Comey testified. "The Russians interfered in our election during the 2016 cycle. They did it with purpose. They did it with sophistication. They did it with overwhelming technical efforts. And it was an active-measures campaign driven from the top of that government." Comey added that, to his knowledge, the Russian meddling did not affect the integrity of the nation's voting or vote tabulation. But he left senators with a warning. These kinds of attacks represent a new normal from the Russians – and they will target whichever party they feel disfavors them. "They will be back," he said.

Third: President Trump is a shady and transactional operator who knows people in his orbit may be culpable in the Russia scandal. In Comey's telling, Trump repeatedly tried to bring the FBI director into his confidence. First he sought to strike a deal: Comey's ongoing employment in exchange for a vow of loyalty; "he [was] looking to get something in exchange for granting my request to stay in the job," Comey testified. Later, Trump buttonholed Comey in the White House, pressuring him to drop the criminal investigation against Michael Flynn. Comey not only found this directive troubling – he treated it as possible evidence of a crime. "Why did he kick everybody out of the Oval Office?" Comey posed to the Senate Intelligence Committee. "That, to me, is – as an investigator – is a very significant fact." Later, Trump pressured the FBI director repeatedly to clear the "cloud" over his administration, including by targeting his campaign associates: "If some of my satellites did something wrong," the president told Comey, "it'd be good to find that out."

Fourth: Comey was the source of memos – shared through an intermediary at Columbia University – that reached the press after his firing. "As a private citizen, I felt free to share" the memos, Comey testified. "I thought it very important to get it out." With unusual candor, Comey testified that it was his hope that publication would force the Justice Department to appoint a special prosecutor, a gambit that succeeded when Robert Mueller was tapped to head up the Justice Department's criminal investigation of the Russia scandal. Mueller, a former FBI director in his own right, now has all of Comey's materials – including the unclassified memos of his encounters with Trump and any classified evidence trail he left behind at the FBI.

Fifth: Comey has no doubt why he was fired. "I take the president at his word, that I was fired because of the Russia investigation," Comey said Thursday. "Something about the way I was conducting the Russia investigation was ... putting pressure on him ... irritating him," Comey added. "That is a very big deal, and not just because it involves me. The nature of the FBI and the nature of its work requires that it not be the subject of political consideration."

While Comey's testimony cast new and troubling light on the Russian affair, his Senate appearance raised big and often troubling questions. This seemed to be by design. The senators asking the questions are – like Comey – privy to classified information that cannot be discussed in public. But many of the provocations posed by Democrats on the Intelligence Committee appeared designed to offer a refracted view of still-classified material – or to give dark weight to Comey's replies that he could not discuss the material in public.

Here are the big questions.

First: Why was Comey so alarmed after his initial January 6th meeting with Donald Trump at Trump Tower? Comey had been tasked with briefing Trump, one-on-one, on the "salacious" details in the so-called Steele dossier. As Comey recalled in his written testimony, he felt compelled to create a record of the encounter "on a laptop in an FBI vehicle outside Trump Tower the moment I walked out of the meeting." Why had Comey come away convinced that Trump was not only a liar, but that the integrity of the FBI could be under threat from the new head of the executive branch? Did the FBI director catch the president in a lie? Comey leaves us a clue: The allegations of the Steele dossier continued to haunt Trump, including in a conversation more than three months later, on March 30th. "He said he had nothing to do with Russia, had not been involved with hookers in Russia, and had always assumed he was being recorded when in Russia," Comey records. Comey deflected a question about his own assessment of the dossier during his testimony: "Not a question I can answer in an open setting, Mr. Chairman."

Second: What does the government know about the "backchannel" communications Trump's son-in-law Jared Kushner is alleged to have established with the Russians during the transition? The following exchange, between Comey and New Mexico Democrat Sen. Martin Heinrich is remarkable because it is ostensibly a dead-end – Comey says he cannot comment on whether the allegation is true – yet the senator describes the allegation in striking detail, while Comey takes time to inveigh in the strongest terms against the threat of such a practice.

Heinrich: "There are reports that the incoming Trump administration ... attempted to set up a sort of back-door communication channel with the Russian government using their infrastructure, their devices or facilities. What would be the risks ... to setting up unauthorized channels with a hostile foreign government, especially if they were to evade our own American intelligence services?"

Comey: "I'm not going to comment on whether that happened in an open setting. But the ... primary risk is obvious: You spare the Russians the cost and effort of having to break into our communications channels by using theirs. And so you make it a whole lot easier for them to capture all of your conversations, and then to use those to the benefit of Russia against the United States."

Third: What does Comey know about Attorney General Jeff Sessions that led him, and the entire senior leadership of the FBI, to cut the AG – their boss – out of the loop after Trump pressured Comey to drop the criminal probe of Michael Flynn? Comey testified that he knew, weeks in advance of Sessions' formal recusal from the Russia investigation, that he should not be involved. Comey told senators that Sessions was "inevitably going to recuse himself" and that his team "were aware of facts that I can't discuss in an open setting that would make [Sessions'] continued engagement in a Russia-related investigation problematic." These classified reasons suggest the public does not yet have a full accounting of the attorney general's involvement with the Russians. A leak after the subsequent classified briefing with Comey and the Senate Intelligence Committee suggests that Sessions may have had a third, yet undisclosed, meeting with Russian ambassador Sergey Kislyak.

Fourth: Despite the departure of the compromised Michael Flynn, is the U.S. still under threat from Trump advisers or cabinet members who might be susceptible to Russian blackmail? Sen. Ron Wyden, an Oregon Democrat who often pushes to make the broad outlines of classified matters transparent to the public, posed this question to Comey in a way that suggests ongoing danger. And Comey did nothing to downplay those fears.

Wyden: "Flynn is gone, but other individuals with contacts with the Russians are still in extremely important positions of power. Should the American people have the same sense of urgency now, with respect to them?"

Comey: "I think all I can say, senator, is ... the special counsel's investigation is very important. Understanding what efforts there were or are by the Russian government to influence our government is a critical part of the FBI's mission ... and you've got the right person in Bob Mueller to lead it. So it's a very important piece of work."

Fifth: Is President Trump now the subject of criminal scrutiny? Trump has taken Comey's testimony that, under the former director's watch at the FBI, the president had not personally been the subject of a criminal or counterintelligence investigation, and run with it – tweeting of his "total and complete vindication."

But what was true at the moment of Comey's firing may no longer be true, in particular given the president's own Russia-related rationale for giving Comey the axe. Indeed, the trail of evidence that Comey had been accumulating on Trump – including the explosive, closely held allegation that Trump attempted to spike the investigation of Flynn, telling Comey "I hope you can let this go" – is now in the possession of special prosecutor Mueller.

Comey made clear that the darkest questions about Trump and the Russians are now in Mueller's sights. Asked whether the Steele dossier was being "reviewed or investigated or followed up on in any way," Comey replied, "It's Director Mueller's ... responsibility now." Asked whether he felt Trump had obstructed justice, Comey answered, "That's Bob Mueller's job to sort that out." Asked, "Do you believe Donald Trump colluded with Russia?" Comey responded, "That's a question I don't think I should answer in an open setting. But that's a question that'll be answered by the investigation."